1. Field of the Invention
This invention relates to excavation apparatus of the type which employs high velocity air to loosen soil and a pneumatic vacuum to remove the loosened soil, and in particular to excavation apparatus having an improved air lance nozzle with a blade-shaped orifice, an improved air lance adapted to be rotated during excavation, and an improved pneumatic vacuum system including a multistage venturi ejector adapted to be fitted on a material collection container.
2. Discussion of Related Art
The concept of vacuum excavation has been discussed in a number of prior patents. U.S. Pat. Nos. 4,776,781, 4,936,031, 5,140,759, and 5,361,855 all disclose pneumatic soil excavation systems in which a jet of air is directed against a mass of soil by a hand-held nozzle to cause the mass to break up, and in which the loosened soil is collected by entraining it in an air flow carried by a pipe or conduit, and depositing the entrained soil at a site away from the excavation.
The theory underlying the concept of vacuum excavation is well-known. Essentially, application of supersonic jets of air causes local fracturing of the soil and rapid release of expanding high pressure air trapped within the soil at the local fracture sites. The fracturing and gas-release properties of the soil are not shared by man-made structures buried within the soil, such as natural gas lines, water pipes, sewer lines, and the like, and thus these structures are unaffected by the supersonic air jets.
Loosening of the soil by local fracturing and rapid expansion of gases trapped in the soil rather than by direct impact means that the air delivery device generates relatively low reaction forces and can be manipulated by a single person. Vacuum excavation therefore increases productivity relative to hand-excavation methods, i.e., shovels, without sacrificing precision, significantly reducing visible alteration of local landscaping or paving. In addition, the use of a high vacuum for material collection causes an effective evacuation of solid material from difficult to reach areas such as beneath or behind pipes, where shovels cannot fit or are difficult to maneuver.
Despite these advantages, however, the conventional vacuum excavation systems have a number of disadvantages that have prevented their widespread use. On the air lance side of the apparatus used in the conventional systems, the disadvantages include difficulties in handling the air lance, which conventionally must be "bounced" up and down to loosen layers of soil across an area of the excavation, and the need for a larger air supply than is available from the type of air compressor commonly used by contractors to operate pneumatic equipment. On the material collection side of the conventional vacuum excavation apparatus, the disadvantages include both the high initial cost of the vacuum generating equipment, and high maintenance costs.
One of the reasons for the large air consumption on the air lance side of the apparatus is the low resistance provided by the conventional cylindrical nozzle or pipe nipple. Because of this problem, nearly all companies currently performing vacuum excavation are forced to use high volume (100 cubic feet per minute (cfm) or greater) high pressure compressed air for soil breakup.
In order for an air lance to be an effective digging tool, the air must exit the lance at supersonic velocity which creates a shock wave in the air, and in order to create a shock wave at the tip of a 1/4 inch pipe nipple, a high volume of high pressure air is needed. The typical air lance consists of 3/4 or 1 inch internal diameter pipe with a reducer and a 1/4 inch internal diameter pipe nipple at the digging end, and is supplied by a vehicle-mounted engine driven air compressor having an airflow rating of 180 cfm or greater. Since the most commonly available compressor has a rating of 185 cfm, the conventional cylindrical air lance requires the full power of the compressor, leaving the compressor unavailable for use as an air supply for a vacuum system, or to power other equipment, thus necessitating a separate engine driven vacuum pump.
On the vacuum collection side of the conventional vacuum excavation apparatus, most vacuum excavation systems employ vacuum pump and engine systems that require use of positive displacement blowers and various stages of filtration or cyclonic separation between the collection container and the positive displacement blowers, as the blowers are very susceptible to internal damage from particulates passing into the motive sections. Motors to drive the positive displacement blowers in the conventional vacuum pump and engine systems vary but generally range between 15 and 50 hp, with some systems making use of power take off linkage from the vehicle on which the unit is mounted.
In addition to being expensive and difficult to maintain, such systems are difficult to transport, and generally can only effectively access locations less than 25 feet from the vacuum source. The V-belts, filters, and internal combustion engines associated with vacuum pump/engine systems require complicated maintenance, which is compounded by the typically dirty and dusty work environments in which they are used, with those systems utilizing truck mounted hoppers being especially difficult to clean. In addition, conventional excavation systems of this type have poor water handling capabilities, since water can contaminate the vacuum generating equipment and especially the filters. Such systems can obviously not be taken indoors or up to work zones in high rise buildings.
While systems have also been proposed which use venturi-type ejectors to generate the vacuum and thereby reduce maintenance costs by eliminating the need for complex filtration or cyclonic separation systems, the conventional venturi systems require high air volumes (450 CFM or more) to generate an effective suction, and therefore require the use of large high cost air compressors and high volume connection hoses, which negates the advantage of simplicity offered in theory by the venturi engine concept.
The ultimate effect of these disadvantages is that, in order to begin using a conventional vacuum excavation system, an initial investment of greater than $100,000.00 is required, with significantly increased operating costs to be expected during the life of the system. This puts the cost of vacuum excavation apparatus out of reach of virtually all private contractors, not to mention others who might benefit from an inexpensive air lance and material removal system. On the other hand, the apparatus of the invention, as described below, currently has a cost of approximately one fourth the minimum cost of the conventional systems, and far lower transportation and maintenance costs.